1. Field of the Invention
The present invention relates to a control method which takes account of changes occurring in the material volume and mass flow of a process to be controlled, at the manipulation of an actuator, like a control valve, for control of another process quantity. The invention also relates to an apparatus for implementing the method.
2. Description of the Related Art
In control of the concentration or other quantity which describes the instantaneous state of the process material, changes of the amount of material contained in the process and changes of the material flow make the control task difficult. For observation of variation of the continuous flow, a method and the corresponding apparatus have been disclosed earlier, and they have received a patent in Finland (59494), United Kingdom (2051424) and U.S. Pat. No. 4,358,821. In the referred patent documents, common disturbances and changes of flow and the benefit gained through their observation are illustrated by examples. In the method disclosed, the material volume of the process has been considered constant and thereby one has completely ignored its possible variation, the disturbances caused by this and incorporation of its variation in control of other process quantities. On the basis of the above method, one cannot either make any obvious conclusions on how changes of material volume could be taken into account in the pursuance of better control results.
However, the amount of material, like the volume of a liquid, varies often in the industrial equipment. Typical examples of this are buffer vessels charged with the task of decreasing the changes which are present in both the quality and the flow of the incoming material. In such a case, both the material volume and flow, in addition to the input concentration, affect the concentrations in the output channel.
The controllers of a production process of variable volume have to be tuned for the worst case occurring in the practice., If e.g. the process exhibits a time delay which is essentially dependent on the volume, the feedback controller of concentration has to be tuned for the greatest volume present in practice. This is so, because the control would get worse and the control loop could run into a state of oscillation, if the volume could become greater than the volume under tuning conditions.
The applicant is not informed of examples of cases in which the variable volume would have been incorporated in control of material properties, the less so of cases in which both the volume and flow would have been incorporated. It may be possible to construct such a control system by means of conventional control components whereby the system would consist of e.g. cascade control loops. An inaccurate operation would be characteristic of such a controller including the fact that the control parameters would almost always deviate from their optimal values.